1. Field of the Invention
The present invention relates to apparatuses and methods for controlling line impedance of a telephone line.
2. Description of the Related Art
Conventionally, line standards for communication apparatuses are provided for each country, and communication apparatuses must be compliant with the line standards of each country. The ringer impedance standard that is one of such line standards provides an upper limit to a ringer impedance value in some countries such as Republic of South Africa. In those countries, elements having different impedance values from each other have been used through a network control unit (NCU) for compliance with the standards of the countries. This may prevent commonality of circuit substrates of communication apparatuses and may therefore increase their costs. In a communication apparatus disclosed in U.S. Pat. No. 6,522,745, a data access arrangement (DAA) is used instead of such an NCU, and the data access arrangement controls a line current value to allow the ringer impedance value to be compliant with the corresponding standard.
However, according to U.S. Pat. No. 6,522,745, a transition of line voltage may occur when a telephone line having a high inductor (L) component is inserted to a communication apparatus or a telephone connected to a communication apparatus is OFF-hooked or ON-hooked. This may cause a problem that the control over ringer impedance by a data access arrangement may oscillate line voltage.
Similarly, when pulse dialing is performed from a telephone connected to a communication apparatus, a transition of line voltage occurs, which may oscillate line voltage. Such oscillation of line voltage may possibly have an adverse effect on a line facility such as exchange.